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The L2TP Tunnel Could Not Be Set up Because of the Low Processing Speed of the LNS

Publication Date:  2012-07-27 Views:  70 Downloads:  0
Issue Description
The ME60 served as the LAC and Vendor C’s 2xxx router as the LNS. After the cutover, the L2TP tunnel could not be established between the ME60 and the peer device. For this case, the tunnel parameters were advertised by the RADIUS server. The user’s device initiated the PPPoE dialing. After the user passed the authentication, the ME60 began to set up the tunnel for the connection with the LNS. 
Alarm Information
The user reported that the L2TP service was unavailable. Running the display l2tp tunnel command on the ME60, the engineer found that the tunnel was not set up. 
Handling Process
1. Running the ping command to check the route, the engineer found no problem with the route between the LAC and the LNS.
2. Running the trace object command, the engineer found that the user passed the authentication and then the ME60 sent the tunnel connection request to the LNS. This means that the user dial-up and authentication proceeded normally. However, in the subsequent tunnel setup process, the ME60 prompted that "Failed to create L2TP session and notify server user down".
3. Checking with the vendor of the RADIUS server, the engineer found that all tunnel parameters advertised were correct.
4. Running the debug vpdn l2x events command on the LNS, the engineer found that the LNS could normally receive the SCCRQ packet from the ME60 and set up the tunnel. However, before the tunnel setup was completed, the LNS received the same SCCRQ packet again. Taking this as abnormal, the LNS stopped setting up the tunnel. Such a process repeated again and again. As a result, the tunnel could not be set up.
Through the preceding operation and analysis, the engineer believed that the C2xxx did not complete the setup of the tunnel before the timeout. Therefore, the ME60 re-initiated the tunnel setup request, causing the C2xxx to stop the setup process. By default, the interval for the ME60 to re-transmit the packet upon timeout is 2s. The engineer ran the following commands to change the default value to 5s. Then the problem was solved.
l2tp-group test
tunnel timeout 5 
Root Cause
Possible causes:
1. No proper route to the LNS.
2. Dial-up users failed to pass the authentication. Therefore, the ME60 did not initiate the tunnel connection request to the LNS.
3. The RADIUS server advertised wrong tunnel parameters.
4. The parameter was not properly set for the interconnection between the LNS and the LAC.